Fluidized catalytic cracking (FCC) processes have been used extensively in the conversion of high boiling portions of crude oils such as gas oil and heavier components customarily referred to as residual oils, reduced crude oils, atmospheric tower bottoms, topped crudes, vacuum resids and the like, to produce useful products such as gasoline, fuel oils, light olefins and other blending stocks. The processing of such heavy feedstocks which comprise very refractory components, e.g. polycyclic aromatics and asphaltenes, and which deposit large amounts of coke on the catalyst during cracking typically require severe operating conditions including high temperatures which in turn have presented problems of exceeding operating limits of plant materials of construction as well as catalyst impairment.
At present, there are several FCC processes available for catalytic conversion of such heavy hydrocarbon feedstocks. A particularly successful approach which significantly diminishes such problems as mentioned above is described, for example, in U.S. Pat. Nos. 4,664,778; 4,601,814, 4,336,160; 4,332,674 and 4,331,533. In such processes, a combination high temperature fluidized catalytic cracking-regeneration operation is provided for the simultaneous conversion of both of the high and low boiling components contained in gas oils and residual oils with high selectivity to gasoline and lighter components, and with low coke production. These high temperature conversion processes have been made possible in part due to the use of two-stage catalyst regeneration processes. In the first stage of such regeneration processes, catalyst particles, which have deposited on them hydrocarbonaceous materials such as coke, are regenerated under conditions of oxygen concentration and temperature selected to particularly burn hydrogen associated with hydrocarbonaceous material. These conditions result in a residual level of carbon left on the catalyst and the production of a carbon monoxide (CO)-rich flue gas. This relatively mild first regeneration serves to limit local catalyst hot spots in the presence of steam formed during hydrogen combustion so that formed steam will not substantially reduce the catalyst activity. A partially regenerated catalyst substantially free of hydrogen in the remaining coke and comprising residual carbon is thus recovered from the first regenerator and passed to a second stage higher temperature regenerator where the remaining carbon is substantially completely burned to CO.sub.2 at an elevated temperature up to 1800.degree. F.
This second stage regeneration is conducted under conditions and in the presence of sufficient oxygen to burn substantially all residual carbon deposits and to produce CO.sub.2 -rich flue gas.
The regenerated catalyst is withdrawn from the second stage and charged to the riser reactor at a desired elevated temperature and in an amount sufficient to result in substantially complete vaporization of the hydrocarbon feed. The catalyst particles are at a temperature typically above 1300.degree. F. and often above 1400.degree. F., such that at the selected catalyst feed rate and hydrocarbon feed rate the vaporizable components of the hydrocarbon feed are substantially completely vaporized rapidly in the riser reactor whereby subsequent catalytic cracking of the feed is accomplished.
As will be appreciated by those skilled in the art, the above-described processes make feasible the high temperature conversion processes required to convert gas oils and residual oils and other high boiling components of crude oils by substantially removing higher temperature restrictions and extending the temperature of regeneration up to 1800.degree. F. if need be without exceeding the metallurgical limits of the regeneration equipment and unduly impairing catalyst activity.
As will also be appreciated by those persons skilled in the art, such FCC processes as described above have the potential capability for maximizing selected product yields, for example, gasoline or light cycle oils (LCO)/distillate, from a given hydrocarbon feedstock. As an FCC unit operation is shifted from a gasoline producing mode, for example, into a maximum distillate producing mode or operation, the LCO yield and cetane quality thereof improves and thus can be used more favorably for blending to form a diesel fuel product. In another embodiment, such processes also have the potential capability of producing large yields of olefins, especially propylene and butylenes, for use as valuable alkylation gasoline charge stock, or in the manufacture of petrochemicals.
It is therefore often desirable to operate such FCC processes in such a manner so as to maximize the production of a given product or products. For example, any or all of the above operations may be relied upon for upgrading a heavy hydrocarbon feedstock, e.g. gas oil and/or residual oil or portions thereof, to produce maximum quantities of fuel oil distillates and diesel fuel at the expense of gasoline or, alternatively, to produce maximum quantities of olefins and other gasoline charge stocks, in order that adequate supplies of such desired products may be available during times of increased demand.
It is known that depending upon the riser reactor cracking severity selected, e.g. reactor outlet temperature (ROT) selected, a significant improvement in LCO/distillate product with a reduction in gaseous product yield can be achieved, or, alternatively, an improvement in light olefin products can be achieved. In particular, it is known that LCO distillate yields can be maximized by restricting a riser outlet cracking temperature to within the range of about 870.degree. F. to about 950.degree. F., and more particularly with the range of about 880.degree. F. to about 970.degree. F. Thus, LCO/distillate and other fuel products production is maximized as conversion of the hydrocarbon feedstock to gaseous product yield including C.sub.3 /C.sub.4 olefins and lower boiling range material is decreased.
It is also known that yields of light olefins can be maximized by operating a riser outlet cracking temperature within the range of about 1000.degree. F. to about 1100.degree. F., and more particularly within the range of about 1020.degree. F. to about 1060.degree. F.
Conversion, which increases with temperature, is normally controlled in FCC processes by the amount of hot regenerated catalyst cycled through the riser reactor in a given amount of time, e.g. catalyst-to-oil ratio. However, decreasing the catalyst-to-oil ratio to restrict the riser cracking outlet temperature and thus the conversion to maximize LCO/distillate production, or to increase the production of light olefins is accompanied by several disadvantages. First, a lower catalyst-to-oil, ratio decreases the rate of catalytic activity. Moreover, in most cases, the riser outlet temperature is essentially determinant of the mix zone temperature, or the theoretical equilibrium temperature which would occur by combining a given ratio of hot regenerated catalyst and hydrocarbon feed thereby leading to vaporization of the feed before catalytic cracking begins. As the mix zone temperature decreases due to a lower riser outlet temperature (lower severity-lower conversion operation), a larger fraction of the hydrocarbon feed may not vaporize upon injection in the riser. This can cause the apparent oil and coke deposition on the catalyst to rise very quickly. Such increased coke deposition is considered to be unnecessary and tends block catalyst cracking sites. Raising the riser temperature to increase the mix temperature is undesirable when, for example, attempting to maximize LCO/distillate production since this promotes undue cracking reactions resulting in a high production of gasoline, and thus the desired selectivity to distillate fuels is lost.
Further, when operating a catalytic cracking operation in a high conversion mode at high riser reactor outlet temperature, for example, to maximize production of C.sub.3 through C.sub.6 light olefinic materials, excessive coking can occur due to polymerization and/or recracking of already heavily reacted light cycle oil and heavy cycle/slurry oil conversion products, and in the production of unwanted diolefins from thermal overcracking, thus detracting from the desired product yield.
In view of the above, it is therefore an object of the present invention to provide an improved version of a combination high temperature fluidized catalytic cracking-regeneration process wherein the production of a desired product or products from catalytic cracking of gas oils or residual oils or mixtures thereof and the like is maximized. More particularly, it is an object of this invention to provide such processes which produce more fuel oil distillates and diesel oil or alternatively more light olefins and gasoline charge stocks than is conventional while avoiding problems associated with restricting riser reactor outlet temperatures or alternatively, those problems which can arise when operating a riser reactor in a catalytic cracking operation at high outlet temperatures such as mentioned above.
It is a further object of the present invention to provide such processes as described above wherein catalyst regeneration is carried out successively in separate, relatively lower and higher temperature, regeneration zones each independently operating under selected conditions.
Additional objects of the present invention will become apparent from the following summary and detailed discussion of preferred embodiments of this invention.